Conventionally, an electric power steering apparatus has been known as one example of an electrical control system to which electrical power is supplied from a battery mounted on a vehicle. This electric power steering apparatus, which controls the amount of electricity supplied to an electric motor in accordance with the operation condition of a steering wheel to thereby impart steering torque, consumes a considerably large amount of electrical power. Therefore, when the capacity of a battery lowers (hereinafter referred to as “deterioration”), a limitation is imposed on the amount of electricity supplied to the electric motor, which generates the steering torque, with the result that the motor may fail to generate a predetermined steering torque, and a drop may occur in the voltage supplied to other electrical control systems operating simultaneously.
Accordingly, it is important to detect deterioration of the battery before such a situation occurs, and to prompt a driver to exchange the battery.
An apparatus for detecting deterioration of a battery mounted on a vehicle is disclosed in, for example, JP2000-190793A. The disclosed apparatus is configured such that current is supplied from a battery to a predetermined load during a period between turning on of a key switch that detects insertion of a start key and turning on of an ignition switch that starts an engine, and the battery condition is diagnosed on the basis of a change in the battery voltage at that time.
Further, an apparatus disclosed in JP2005-28900A includes two electric motors for steering wheels, and diagnoses the battery condition on the basis of a drop in the battery terminal voltage at the time when a large current is supplied from the battery to the electric motors.